The co-pending International Application published as WO 2010/083854, which is incorporated herein by reference, discloses a self-cleaning window arrangement for separating atmospheric pressure from vacuum while letting X-ray radiation pass through. The window arrangement has heating means for cleaning an inner surface, facing the vacuum, in order to evaporate a contaminant during operation. In particular, the window can be cleaned from splashes, droplets and depositing mist from the liquid-jet anode.